Undersized tires!!

I finally found out why when I am going 65 according to my speedometer, which is the speed limit, everyone is flying by me... I have undersized tires!

This is really pissing me off. I took my GPS around town this evening and when my speedo reports 40 MPH I am really going 36 MPH. I did not have a chance to get up to 65 but I bet that speed is really fast compared to what I am traveling.

I am supposed to be running 235/75 R17s and I have 245/65 R17s. According to the conversion done on some other web page, the stock size is 30.9 inches, and the size I am running is 29.8. A full inch smaller.

Will undersized tires cause any adverse effects on my Truck? They don't have much tread left on them so by next spring I'll be getting new ones anyway, for now I need to figure out at what speeds I am going on the dash to what they really are on the street.

I guess there are two pluses with tires that are shorter than stock...do the speed limit (according to your speedometer) and you'll never have to worry about a speeding ticket and it should feel like it has slightly more power.

I guess there are two pluses with tires that are shorter than stock...do the speed limit (according to your speedometer) and you'll never have to worry about a speeding ticket and it should feel like it has slightly more power.

A 1" difference in tire size will cause about a 3%, not a 10%. Your speedo was off from the factory (no surprise). The dealer should be able to reflash the computer to correct, or better yet, get yourself a programmer - you can improve your mileage (I gained 2 mpg with SuperChips) and get more power.

My speedo never matches my GPS and my tires are stock size. My GPS is always 3 to 4 km/h slower. Tire size is definitely a factor but if the GPS is portable you might want to try it in another vehicle and compare.

I was just thinking about the percentages and how off it would be, so thanks for clarifying that. I went to Ford a while back and they want $75 to program the speedo and I said I think I'll wait.

I had a superchips micro tuner on my 2001 and I loved it, I may look into getting another one for this truck. Also on the 2001, the GPS and the Speedometer were only 1 MPH off over the entire range of speeds 5-95 MPH. I tested it!

95 was hard to sustain with out worry of getting pulled over so that didn't last long, but it read accurately. I'll start looking for programmers and see what the money flow states as to purchasing.

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